Interpretive Summary: Soilborne plant pathogens are major yield-limiting factors in the production of food, fiber and ornamental crops. Take-all, caused by the fungus Gaeumannomyces graminis var. tritici (Ggt), is the most important root disease of wheat in the U.S. and worldwide. Interest in the use of microorganisms to suppress soilborne plant pathogens has been stimulated in recent years by trends in agriculture toward greater sustainability and public concerns about hazards associated with the use of chemical pesticides. Take-all can be suppressed by a variety of soil microorganisms including saprophytic fungi such as Trichoderma spp. and bacteria such as fluorescent Pseudomonas spp that are associated with the roots of wheat. Strains of the take-all pathogen that have lost virulence (hypovirulent) also can provide biocontrol because they colonize sites on wheat roots normally colonized by the virulent pathogen without causing disease; thus, they preempt establishment of virulent strains. The purpose of this research was to assess the feasibility of combining fluorescent Pseudomonas spp. with avirulent Ggt in order to enhance biocontrol activity. Although no consistent improvement in biocontrol activity was achieved with the combinations, the authors demonstrated that each biocontrol agent is effective at different stages of take-all development. These findings are useful to ongoing efforts to develop commercially viable biocontrol agents for plant pathogens.

Technical Abstract:
Hypovirulent Gaeumannomyces graminis var. tritici strain L108hv, five strains of fluorescent Pseudomonas spp., and a mixture of four of these bacterial strains were used individually and in combination for the suppression of take- all of wheat. In the growth chamber, L108hv and all five individual bacterial strains provided a significant reduction in take-all severity compared with a nontreated control. Combination of L108hv and the bacteria did not significantly enhance the suppressiveness of either agent. In the field at Pullman, WA, L108hv significantly reduced crown root infection of winter wheat but not seminal root infection while the reverse was true for mixture of bacterial strains suggesting these agents may be more effective at different stages in disease development. In a second field trial a Pullman, yield of spring wheat was increased by 20.8%. In contrast, in two field trials a Mt. vernon, WA, L108hv had no effect on take-all of spring wheat. We conclude that specific biocontrol agents may be better for certain environments and that biocontrol treatments should be customized for the target site.